Water heater

ABSTRACT

A water heater having a water tank and multiple electric resistance heating elements extending into the water tank for heating water in the tank. The water heater includes a proportional band temperature controller for conducting electric power to the electric resistance heating elements in bursts. Each burst of electric power is followed by a period during which the temperature controller does not conduct power to the electric resistance heating element. In one embodiment, each burst of electrical power lasts for about 95% or less of a cycle comprised of one burst of electric power followed by the period during which the temperature controller does not conduct electric power. Further, activation of the heating elements by the controller is carried out in a sequential or other timed or controlled fashion to permit uniform heating of the water in the tank.

RELATED APPLICATIONS

[0001] This is a continuation-in-part of U.S. Ser. No. 09/361,825, filedAug. 17, 1999.

FIELD OF THE INVENTION

[0002] The present invention relates generally to electrical waterheaters. More particularly, the invention relates to methods andapparatus for pulsing electrical power to electrical resistance heatingelements in a manner to improve the heating efficiency of the waterheater.

BACKGROUND OF THE INVENTION

[0003] A storage-type water heater typically comprises a permanentlyenclosed water tank, a cylindrical shell coaxial with and radiallyspaced apart from the water tank to form an annular space between theouter wall of the water tank and the inner wall of the shell, andinsulating material in at least a portion of the annular space forproviding thermal insulation to the water tank. The water tank hasvarious appurtenances such as inlet, outlet and drain fittings.Additionally, the water heater is provided with a water heating andtemperature control system. The water heating and temperature controlsystem includes an electrical-resistance-heating element. The heatingelement extends through a fitting in the wall of the water tank suchthat the heating element is inside the tank. The heating element isconnected to an electrical power source outside the water tank.

[0004] Conventional water heating and temperature control systemstypically further include a mechanical thermostat. The mechanicalthermostat closes a switch to allow electrical power through theelectrical resistance heating element when water in the tank is sensedto be below a selected set-point temperature, and opens the switch tostop electrical power from passing through the electrical resistanceheating element when the water in the tank is at or above the set pointtemperature. Electrical power through the electrical resistance heatingelement is either fully on, passing full electrical current, orcompletely off. Due to variations in manufacture and hysteresis of themechanical thermostat, the temperature of the water will “overshoot” thedesired set-point temperature. In other words, the water heating andtemperature control system allows the electrical resistance heatingelement to continue heating water in the water tank even when the watertemperature is above the set point temperature. It would be beneficialto prevent or limit the amount of overshoot of the conventional waterheater.

SUMMARY OF THE INVENTION

[0005] Accordingly, the invention provides a water heater having acontroller for modulating electric power to anelectrical-resistance-heating element in controllable pulses or bursts.Providing electric power to the heating element in pulses or burstsallows an equal amount of water to be heated to a selected temperatureat substantially the same rate as with a mechanical temperaturecontroller of the prior art, yet uses substantially less electric powerto heat the water. Therefore, modulating the electric power improves theefficiency of the water heater.

[0006] A preferable way for modulating electric power in short bursts tothe resistance heating element is by using a proportional bandtemperature controller that takes into account the unique signature ofthe water heater. That is, when calculating the amount of modulationbetween a burst of electric power being supplied to a heating elementand a period during which no electric power is supplied to the element,the water heater may vary the amount of modulation based on a number ofvariables or water heater characteristics. The variables may include,but are not limited to: heating element wattage, element watt density,location of the heating element(s), number of elements mounted within atank, operating voltage of the water heater, inlet water temperature,water capacity of the water tank, ambient room temperature of thephysical environment in which the heater is installed, and usagepatterns of the facility in which the heater is being used. By combiningall of these aspects with proportional band technology, significantlygreater energy savings are achieved over conventional electric waterheaters.

[0007] The invention further provides a water heater including a tankfor holding water, a water inlet line having an inlet opening thatintroduces cold water to the tank, a water outlet line having an outletopening that withdraws heated water from the tank, and a first heatingelement extending into the tank. The water heater further includes acontrol circuit operable to control the supply of electric power to aheating element in bursts. Each burst is followed by a period duringwhich electric power is not supplied to the heating element.

[0008] In one embodiment, the tank has a tank characteristic, theheating element has an element characteristic, and the control circuitincludes a temperature sensor operable to sense a temperature of thewater within the tank. The control circuit further includes a controllerin communication with the heating element and the temperature sensor.The controller is operable to receive the sensed temperature from thetemperature sensor, to calculate a heating strategy for the water heaterbased in part on the element characteristic and/or the tankcharacteristic, and to generate a signal activating the heating elementin response to the heating strategy. In another embodiment, the controlcircuit is further operable to change the proportion of on to off timein response to the sensed water temperature and at least one of anelement characteristic, a tank characteristic, an external water tanktemperature, a water consistency, and a history of water use.

[0009] The invention even further provides a method of controlling atemperature of water in a water heater. The method includes the acts ofdetermining an element characteristic of the heating element, sensing atemperature of the water in the tank, calculating an amount of power tobe provided to the heating element based at least in part on the elementcharacteristic and water temperature, and transmitting the amount ofpower from the power source to the heating element. The calculating actmay also be based at least in part on a tank characteristic, anenvironment (i.e., ambient) temperature, or a water characteristic(i.e., temperature, hardness, etc.).

[0010] The invention further provides a software program for generatinga signal resulting in an amount of power to be transmitted to a heatingelement. The software program generates the signal by obtaining a waterheater code from a memory unit. The water heater code is based at leastin part on a heating element characteristic or on a tank characteristic.The software program further includes receiving the temperature of theliquid from the temperature sensor, calculating the amount of power totransmit to the heating element based at least in part on the waterheater code and the sensed temperature, and generating the signalresulting in the amount of power being provided to the heating element.

[0011] Other features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdetailed description, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a sectional view of a water heater embodying theinvention, and showing the arrangement of the temperature controller ofthe present invention in relation to other components of the waterheater.

[0013]FIG. 2 is an electrical schematic of a temperature controllerembodying the present invention.

[0014]FIG. 3 is a plot of energy usage data of a mechanical temperaturecontroller of the prior art and a proportional band temperaturecontroller of the present invention.

[0015]FIG. 4 is a plot of energy consumption rate data of the mechanicaltemperature controller of the prior art and the proportional bandtemperature controller of the present invention.

[0016]FIG. 5 is a sectional view of another water heater embodying theinvention and having multiple heating elements.

[0017]FIG. 6 is a sectional view of yet another water heater embodyingthe invention and having multiple heating elements.

[0018]FIG. 7 is a partial sectional view of the water heater shown inFIG. 6.

[0019]FIG. 8 is a sectional view of a water heater including acontroller embodying the invention.

[0020]FIG. 9 is an enlarged partial view of the controller shown in FIG.8.

[0021]FIG. 10 is a schematic representation of the control circuit shownin FIG. 8.

[0022]FIG. 11 is an electrical schematic of a power supply for thecontrol circuit shown in FIG. 10.

[0023]FIG. 12 is an electrical schematic of a zero crossing detector ofthe control circuit shown in FIG. 10.

[0024]FIG. 13 is an electrical schematic of a low-voltage reset circuitof the control circuit shown in FIG. 10.

[0025]FIG. 14 is an electrical schematic of a temperature sensingcircuit of the control circuit shown in FIG. 10.

[0026]FIG. 15 is an electrical schematic of a thermostat of the controlcircuit shown in FIG. 10.

[0027] FIGS. 16(a) and 16(b) are an electrical schematic of portions ofthe control circuit depicted in FIG. 10.

[0028]FIG. 17 is an electrical schematic of an oscillator for thecontrol circuit shown in FIG. 10.

[0029]FIG. 18 is a flowchart representing a method of controlling thewater heater shown in FIG. 8.

[0030]FIG. 19 is a flowchart representing an exemplary method forperforming a test to determine whether a heating element is submerged.

[0031]FIGS. 20a, 20 b, 20 c and 20 d are portions of a flowchartrepresenting an exemplary method of performing the acts of gatheringsensor samples, computing water temperature, computing a thermostatsetting, changing operating mode if necessary, setting a heating cyclestate, and setting a heating priority.

[0032]FIG. 21 is a flowchart representing an eight hundred microsecondinterrupt event.

[0033] Before one embodiment of the invention is explained in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangements of the componentsset forth in the following description or illustrated in the drawings.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways. Also, it is understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including” and “comprising” and variations thereof herein is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items. The use of “consisting of” and variations thereofherein is meant to encompass only the items listed thereafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034] As described above, the use of a proportional band temperaturecontroller in a water heater having an electrical resistance heatingelement has the unexpected advantage of heating water in the waterheater to a preselected set point temperature while consuming lesselectric power than is consumed when heating the same amount of water tothe same set point temperature in the same water heater using amechanical temperature controller of the prior art.

[0035] An exemplary proportional band temperature controller is anelectronic device which comprises a water temperature sensing device(thermistor), a temperature set point device (variable rheostat), agated thyristor for switching electric power to the resistance heatingelement, and a logic circuit for controlling the thyristor in responseto signals from the water temperature sensing device and the temperatureset point device. The logic circuit receives a voltage input from thewater temperature sensing device and the temperature set point devicewhich indicates the differential of the water temperature from the setpoint temperature. The logic circuit, in response to the voltage inputsfrom the water temperature sensing device and the temperature set pointdevice, signals the gated thyristor. At large temperature differentialsbetween the water temperature sensing device and the temperature setpoint device, the logic circuit signals the gated thyristor to conductelectricity during a major portion, about 94%, of each cycle of the ACcurrent, and signals the gated thyristor to stop conducting electricityfor about 6% of each AC cycle. As the temperature differential betweenthe water and the set point narrows, the water temperature enters aproportional control band where the logic circuit begins to exert morecontrol over the gated thyristor to limit electric power to theresistance heating element. As the water temperature enters theproportional control band, the logic circuit establishes a new controlcycle period and signals the thyristor to conduct electric power for 85%of each cycle and to stop conducting for 15% of each cycle. As the watertemperature gets closer to the set point temperature the logic circuitsignals the thyristor to conduct for less of each cycle period. When thewater temperature reaches the set point temperature, the logic circuitcloses the thyristor and electric power is not supplied to theresistance heating element until the water temperature again falls belowthe set point temperature. To prevent undue cycling about the set pointtemperature, the logic circuit is set to require the water temperatureto drop 5° to 10° F. below the set point temperature before thethyristor is again signaled to conduct electric power and heat the waterback to the set point temperature.

[0036] This improvement in the efficiency of heating water in the waterheater using a proportional band temperature controller is notcompletely understood. In theory, essentially all the electrical powersupplied to a resistance heating element will be converted to heat, andthat heat should be transferred to the water surrounding the resistanceheating element. The same amount of electric power should heat the sameweight of water the same number of degrees temperature. As shown in theexample below, a water heater having a proportional band temperaturecontroller requires about 10% less electrical power to heat a tank ofwater to a selected set point temperature than the same water heaterhaving a mechanical temperature controller of the prior art. Theimproved accuracy of a proportional band temperature controller forbringing water to a set point temperature with little overshoot accountsfor some of the improved efficiency over a mechanical temperaturecontroller, but does not appear to account for all.

[0037] While not wishing to be bound, I suggest that the improvement inheating efficiency when using a proportional band temperature controllerarises from physical conditions within the water tank affecting thetransfer of heat from the resistance heating element to the water. Aproportional band temperature controller conducts electric power to theresistance heating element in short bursts followed by short periodsduring which electric power is not conducted until the water in thewater tank reaches a selected set point temperature. The proportionalband temperature controller accurately stops conducting electric powerto the resistance heating element when the water reaches the set pointtemperature. On the other hand, a mechanical temperature controller ofthe prior art conducts electric power to the resistance heating elementcontinuously at full power as the water is heating. When the waterreaches the set point temperature mechanical characteristics of thebimetallic thermocouple may cause the mechanical temperature controllerto overshoot and heat the water to a temperature above the set pointtemperature before it stops conducting electric power to the resistanceheating element.

[0038] A resistance heating element, as is used in domestic waterheaters, heats in a few seconds to a temperature in the range of 800° F.to 900° F. Water, in contact with such a hot resistance heating element,may vaporize depending on tank pressure, may form a layer of vaporaround the resistance heating element and reduce the transfer of heatfrom the resistance heating element to the water. With a mechanicaltemperature controller, the resistance heating element is so heated andremains at a high temperature until the bimetallic thermocouple cuts offelectric power. Heat from a resistance heating element controlled by amechanical temperature controller may be radiated to the wall of thewater tank, or may be transported by vaporization convection currents tothe top of the water tank where the excess heat is absorbed in thetopmost layer of water which is located away from the temperaturesensing bimetallic thermocouple.

[0039] With a proportional band temperature controller, the resistanceheating element is heated during each burst of electric power and iscooled by contact with the water during periods between bursts. Thiscooling of the resistance heating element between each burst of electricpower reduces the temperature to which the resistance heating element israised and reduces the potential for accumulation of vaporization aroundthe hot resistance heating element. Consequently, heat transfer from theresistance heating element to the water is increased. Supplying electricpower to a resistance heating element in a water heater in discreteshort bursts, each burst followed by a period with the electric powershut off, improves the efficiency of heat transfer from the resistanceheating element to the water in the water heater.

[0040] Proportional band temperature controllers are well known andwidely used in many commercial applications, including to control watertemperature in such appliances as coffee makers. Proportional bandtemperature controllers have not, to my knowledge, been used to controlthe temperature of a large volume of water in a storage water heater.

[0041]FIG. 1 of the drawing shows a sectional view of a water heater 10comprising a permanently enclosed water tank 11, a shell 12 surroundingwater tank 11, and foam insulation 13 filling the annular space betweenwater tank 11 and shell 12. Water inlet line or dip tube 14 and wateroutlet line 15 enter the top of water tank 1 1. The water inlet line 14has an inlet opening 22 for adding cold water near the bottom of watertank 11. Water outlet line 15 has an outlet opening 24 for withdrawinghot water from near the top of water tank 11. Resistance heating element16 extends through the wall of water tank 11. The proportional bandcontrol circuitry in control box 17 is connected to resistance heatingelement 16. Thermistor 18, in contact with the outer wall of water tank11 for sensing the temperature of water in water tank 11, is connectedto the logic circuit by electrical wire 19. Electric A.C. power issupplied to the gated thyristor through line 20. A customizable operatorinterface may be mounted on the outside of the water heater to permitcommunication with the control box 17 and provides security protectedaccess for control of the heating element. The operator interface may beoperable to provide direct or remote control of the heating element.

[0042]FIG. 2 of the drawings is a schematic drawing of a preferredproportional band temperature control circuit 100 for heating water in awater heater according to the method of the present invention. In FIG.2, resistance heating element 125 is a 4,500 watt heating element forheating water in a water heater. Temperature set point device 101 is avariable rheostat for setting the temperature set point in the range ofabout 90° F. to 180° F. Thermistor 102 is for sensing temperature ofwater in the water heater. In an alternative embodiment, a plurality ofthermistors could be placed through the tank to measure watertemperature at a plurality of locations. The output of the thermistorscould be averaged.

[0043] Gated thyristor 103 is a TRIAC, manufactured by Motorola, Inc.,for controlling electric power to resistance heating element 125. Logicchip 104 is a proportional band temperature controller UAA1016Amanufactured by Motorola, Inc. Two hundred forty volt electric power issupplied to the proportional band temperature control circuit 100through lines 105 and 106. Opto-electric coupler 108, as will bedescribed below, is for controlling the amount the water temperaturemust decrease from the set point temperature before the proportionalband temperature control circuit will reactivate.

[0044] A stabilized supply voltage of about −8 Volts is delivered to theproportional band temperature control circuit from line 106 throughZener diode 107 and resistor 109 into line 110. Voltage drops throughtemperature set point device 101 and temperature sensor 102 produce asignal voltage at point 111. The signal voltage is proportional to thetemperature difference between the set point temperature and the sensedwater temperature. The sensed voltage is transmitted via line 112 to oneleg of a voltage comparator 113 within logic chip 104. A referencevoltage, the magnitude of which is determined by voltage drops throughresistors 114 and 115, is generated at point 116. A saw tooth voltage,generated in saw tooth generator 118 in logic chip 104, is imposed uponthe reference voltage at point 119. The reference voltage, modified bythe saw tooth voltage passes via line 117 to the second leg of voltagecomparator 113.

[0045] The saw tooth voltage imposed upon the reference voltage causesthe voltage at the second leg of voltage comparator 113 to vary, in asaw tooth pattern, over a cycle of about 0.85 seconds from a minimum toa maximum voltage. In voltage comparator 113, the signal voltage at thefirst leg is compared to the modified reference voltage at the secondleg. The comparison result is transmitted via line 120 to logic circuit121. In logic circuit 121, a signal is generated for passing via line122, amplifier 123 and line 124 for controlling thyristor 103. When thesignal voltage at the first leg of comparator 113 is greater than themaximum value of the reference voltage at the second leg of comparator113, the signal to thyristor 103 is to conduct and allow electric powerto flow through resistance heating element 125 for heating water in thewater tank. Logic chip 104 is arranged such that the signal in line 124causes thyristor 103 to conduct electricity for 96% of each AC currentcycle and stop conducting for 4% of each current cycle.

[0046] The signal voltage at the first leg of voltage comparator 113will fall to a value less than the maximum value of the referencevoltage at the second leg of voltage comparator 113 as the watertemperature sensed by temperature sensor 102 approaches the set pointtemperature selected on set point temperature device 101. When thesignal voltage is in the range between the maximum value of thereference voltage and the average of the reference voltage value, thetemperature control circuit 100 is within the proportional band controlrange. Thus, when the signal voltage is greater than the value of thereference voltage at the second leg of the voltage comparator, logiccircuit 121 signals amplifier 123 to signal thyristor 103 to conductelectric power to resistance heating element 125. Then, as the saw toothvoltage causes the reference voltage at the second leg of voltagecomparator to increase to a value greater than the value of the signalvoltage at the first leg of the voltage comparator, logic circuit 121signals amplifier 123 to signal thyristor 103 to stop conductingelectric power to resistance heating element 125. As the signal voltageat the first leg of voltage comparator approaches closer to the averagevalue of the reference voltage at the second leg of voltage comparator113, thyristor 103 is not conducting for greater percentages of eachcycle of the generated saw tooth voltage. When the water temperaturesensed by temperature sensor 102 is equal to the set point temperatureof temperature set point device 101 the signal voltage at the first legof voltage comparator 113 will equal the average reference voltage valueat the second leg of voltage comparator 113 and logic circuit 121signals amplifier 123 to turn off thyristor 103, shutting off electricpower to resistance heating element 125. Thyristor 103 remains in thenon-conducting state until the water temperature sensed by temperaturesensor 102 falls below the set point temperature by a preset amount, asis described below.

[0047] The signal voltage at the first leg of voltage comparator 113 andthe reference voltage at the second leg of voltage comparator 113 musthave values which allow logic circuit 121 to produce a signal toamplifier 123 which will properly control thyristor 103 to heat thewater to the desired temperature. Temperature set point device 101 is avariable rheostat the resistance of which may be adjusted manually forchanging the set point temperature. Temperature sensor 102 is athermistor in which the resistance decreases as the sensed temperatureof the water increases. The values of resistors 126 and 127 are selectedsuch that the signal voltage at point 111 will be proportional to thedifference between the set point temperature and the sensed watertemperature. The reference voltage at point 116 is determined by thevalue of resistors 114 and 115, and the magnitude of the saw toothvoltage imposed upon the reference voltage at point 119 is determined bythe values of resistors 128 and 129. The values for these resistors mustbe adjusted to accommodate the characteristics of the particulartemperature set point device 101, temperature sensor 102 and logic chip104 selected for the proportional band temperature control circuit 100.

[0048] As described above, opto-electric coupler 108 is included inproportional band temperature control circuit 100 to prevent unduecycling of thyristor 103 when the sensed water temperature is at aboutthe set point temperature. When the sensed water temperature equals theset point temperature, logic circuit 121 signals amplifier 123 to cutoff thyristor 103 and stop conduction of electric power to resistanceheating element 125. Without opto-coupler 108, when the sensed watertemperature drops a small amount, for example, less than 1° C., belowthe set point temperature, logic circuit 121 will signal amplifier 123to open thyristor 103 and conduct electric power to resistance heatingelement 125 until the sensed water temperature is again heated to theset point temperature. This action results in rapidly turning thyristor103 off and on, to control the sensed water temperature as closely aspossible to the set point temperature.

[0049] Opto-electric coupler 108, connected electrically acrossresistance heating element 125 by lines 130 and 131, operates to makethe sensed temperature appear to be about 5° C. higher than it actuallyis when electric current is flowing through resistance heating element125. So, when the water temperature sensed by temperature sensor 102reaches the set point temperature, thyristor 103 is stopped fromconducting electric current through resistance heating element 125 andopto-electric coupler 108. With no current flowing through opto-electriccoupler 108, the signal voltage at point 111 is determined by voltagedrop through temperature sensor 102 and voltage drop through set pointdevice 101, resistor 126, and resistor 127. Resistor 127 produces avoltage drop equivalent to the voltage drop caused by about a 5° C.temperature change in the sensed temperature. Consequently, the sensedtemperature appears to be about 5° C. higher than it actually is, andthe sensed temperature must drop an additional 5° C. before the signalvoltage at the first leg of voltage comparator 113 will indicate thatthe sensed temperature is below the set point temperature. When voltagecomparator 113 signals logic circuit 121 that the sensed temperature isbelow the set point temperature, logic circuit 121 signals amplifier 123to open thyristor 103 and allow electric current to flow throughresistance heating element 125. With electric current flowing throughresistance heating element 125, electric current flows throughopto-electric coupler 108 via lines 130 and 131. With electric currentflowing through opto-electric coupler 108, resistor 127 is bypassed andthe 5° C. bias to the apparent sensed water temperature is removed.Logic circuit 121 then signals amplifier 123 to open thyristor 103 untilthe sensed water temperature again reaches the set point temperature.This action of opto-electric coupler 108 allows the sensed temperatureto fall about 5° C. below the set point temperature before thyristor 103again conducts electric power through resistance heating element 125,and allows the sensed water temperature to be heated to the set pointtemperature before electric power is cut off from resistance heatingelement 125. This action prevents cycling of electric current throughresistance heating element 125 when the sensed water temperature is atabout the set point temperature.

[0050] In an alternative embodiment, the temperature control circuit 100could include a programmable real time clock wherein peak or off-peakenergy demand periods or vacation operation cycles could be programmedinto the control cycle for the heating element. Additionally, a pressuresensor, temperature sensor, mineral deposit sensor and/or sensor fordetecting the presence of water could be added. The control circuitwould be programmed to disconnect power from the water heater and/or theheating element when predetermined conditions or limits are detected.Further, the control circuit could include means for automaticallyadjusting the set point in response to various conditions such as amountof water used, or whether it is a peak or off-peak energy demand period.

EXAMPLE

[0051] In a first example, an electric water heater having a 4,500 Wattresistance heating element was operated for heating water from 60° F. to120° F. using 240 Volt AC current. In a first run, a commerciallyavailable bimetallic thermostat, as described in the introduction tothis application, was used to sense the water temperature and controlelectric current to the resistance heating element. In a second run, theproportional band temperature control circuit, as shown in FIG. 2 anddescribed in this application, was used to sense the water temperatureand control flow of electric current to the resistance heating element.Results of the two comparative runs are shown in FIG. 3 of the drawings.

[0052] For Run 1, tension on a bimetallic thermostat was adjusted with athreaded stud such that the bimetallic thermostat would snap from a flatconfiguration to a domed configuration at a set point temperature of120° F. The bimetallic thermostat was placed in contact with the outerwall of the water heater water tank at a position about three inchesabove the electric resistance heating element. The bimetallic thermostatwas connected, via an insulating rod, to an electric switch in a linesupplying electric power to the resistance heating element. The watertank was filled with 60° F. water and the electric power connected tothe line supplying the resistance heating element. The bimetallicthermostat remained in a flat position and the electric switch wasclosed. Electric current passed through the resistance heating elementat a rate of 19.7 amperes for about 27 minutes until the water washeated to about 122° F. The bimetallic thermostat then snapped into adomed shape, activating the switch to cut off electric current to theresistance heating element. A graph of water temperature versus time forthis first run is shown in FIG. 3.

[0053] For Run 2, a proportional band temperature control circuit, asshown in FIG. 2 and described above in this application, was used. Thetemperature set point device 101 was calibrated for a set point of 120°F., and the thermistor temperature sensing device 102 was attached tothe water tank about three inches above the resistance heating element125. Thyristor 103 was connected to resistance heating element 125. Thewater tank of the water heater was drained and refilled with 60° F.water and the proportional band temperature control circuit 100 wasconnected to the electric power main. The proportional band temperaturecontrol circuit 100 initially supplied 18.8 amperes of electricity tothe resistance heating element 125, i.e. about 95% of the amperessupplied by the mechanical thermostat of Run 1. After about four minutes(at 68° F.), the proportional band temperature control circuit 100reduced the electricity supplied to resistance heating element 125 to18.6 amperes, i.e. about 91% of the amperes supplied by the mechanicalthermostat of Run 1. After about 21 minutes (at 104° F.), the sensedwater temperature entered the proportional band temperature range andthe proportional band temperature control circuit 100 began to slowlyreduce electric current to resistance heating element 125, until after27 minutes the sensed water temperature reached the set pointtemperature and the proportional band temperature circuit 100 shut offelectric current to the resistance heating element 125.

[0054] Inspection of FIG. 3 shows that the same amount of water washeated to substantially the same temperature in the same amount of timein Run 1 and Run 2. However, in Run 1, 19.7 amperes of electricity wererequired and in Run 2, only about 18.6 amperes of electricity wererequired over the heating period. That is, heating water in a waterheater equipped with the proportional band temperature control circuitof the present invention, which supplies electricity to the resistanceheating element 125 in short bursts followed by short periods withelectricity shut off, requires about 9% less electric power than heatingthe same amount of water to the same temperature in the same waterheater, but using a mechanical temperature controller. This is anunexpected result.

[0055] The pulsing of current to the load by the proportional bandtemperature control circuit permits the water temperature to minutelyrise and fall rapidly in response to the applied current. A briefinterruption of current applied to the heater element each cycle allowsfor a more efficient transfer of radiation energy to the water from theheater element.

[0056] As a second example, a test was performed to determine the actualamount of energy a consumer would use during a typical hot water heateroperating cycle. Referring to FIG. 4, the actual kilowatt hours (kWh) isplotted versus time for a mechanical thermostat and an electronicthermostat including proportional band control logic.

[0057]FIG. 4 illustrates that during a typical heating cycle,approximately 3% less energy is being used as a direct result of usingthe proportional band control logic. It is possible that this percentagecould be increased to approximately 5-5.5% by changing the conductionangle of the triac's firing quadrants, without adversely affecting theperformance of the water heater.

[0058] Additionally, by limiting the current to the heater element usingproportional band control logic and by supplying the current to theheater in pulses, gradually coasting to the temperature set pointwithout overshooting the desired temperature offers an additional 15%energy reduction.

[0059] The combination of current modulation and preventing theovershooting of the temperature set points offers the consumer acombined energy savings of nearly 10% over the cost of operation of asimilar heater using a bimetal mechanical thermostat.

[0060] Overheating water past a reasonable temperature of 125° F. -130°F. generally wastes energy. A typical two inch thick layer of insulationloses its capacity to effectively retain heat at temperatures above 130°F. or so. This energy loss in standby mode is wasteful and potentiallycauses the heater to cycle more often than necessary.

[0061] The proportional band control circuit of the present inventionprevents overshooting and allows the water temperature to drop only 10°F. or so to cycle only the needed difference to return the watertemperature to a desired setpoint.

[0062] An additional advantage to the proportional band control circuitis its suitability for a flammable vapor environment. For example, suchan environment may exist in a garage, workshop, or basement storage areawherein solvents, gasoline, propane or other highly flammable orexplosive vapors are present. Mechanical thermostats and contact typeswitching devices can arc when an electrical contact is made or broken,depending on the amount of current being switched. The electrical arccan ignite a flammable vapor if the vapor is sufficiently volatile. Incontrast, the proportional band control circuit is totally solid state,has no moving parts, and would not ignite flammable vapors.

[0063] While implementing proportional band control as described aboveis advantageous, even greater heating efficiency can be achieved in awater heater with multiple, controlled heating elements. An exemplarywater heater 150 with such elements is shown in FIG. 5, and is thesubject of U.S. Pat. No. 09/361,825, entitled PROPORTIONAL BANDTEMPERATURE CONTROL FOR MULTIPLE HEATING ELEMENTS, filed Aug. 17, 1999,which is incorporated herein by reference. The water heater 150 sharesmany common elements with the water heater 10, and common elements aredesignated with the same reference numerals in FIGS. 1 and 5. However,unlike the water heater 10, the water heater 150 has multiple heatingelements 16 and 16′. Heating element 16 is located in the lower portionof the tank and heating element 16′ is located in the upper portion ofthe tank. The heating element 16′ may be controlled by control circuitrystored in a control box 17′ which receives input from a thermistor ortemperature sensor 18′ through a communication link 19′, such as anelectrical wire. Alternatively, although not shown, the sensor 18′ andheating element 16′ could communicate with control circuitry stored inthe control box 17 and just one controller rather than multiple circuitscould be employed. Communication between the sensor 18′ and heatingelement 16′ could be accomplished through a communication link (notshown) running physically parallel to line 20. In the case ofcontrolling two heating elements with a single controller, the controlcircuitry in box 17 might take the form of a programmablemicroprocessor. Of course, more than two heating elements could beinstalled in the water heater 150 and controlled by such a controller,if desired.

[0064] Regardless of the exact control circuitry used, or whether asingle controller or multiple controllers are implemented, the heatingelements in FIG. 5 are activated sequentially or at some predeterminedfrequency or fashion so that heat energy being transferred to the tank150 is distributed in a balanced or uniform manner. Thus, for example,the heating element 16 might be active for a first period of time T1during which power is supplied to it in the pulsed or multiple-burstmanner described above. Subsequently, the element 16′ might be activatedin a pulsed manner for a period of time T2. Times T1 and T2 may or maynot be of equivalent lengths, and may or may not overlap one anotherdepending on the specific heating application and conditions. Moreover,feedback mechanisms employing the temperature sensors 18 and 18′ may beused to trigger activation of the specific heating elements dependingupon the temperature sensed in the upper and lower portions of the tank11.

[0065] Whatever specific sequencing is employed, the use of aproportional band temperature controller to control multiple elements ina water heater helps to avoid uneven heating of the water in the tank.Uneven heating generally occurs in conventional heating systems wherethe bulk of water heating is accomplished with a heating elementpositioned near the bottom of the heater tank. This configuration oftenresults in the creation of “stacking,” where water that is heated risesto the top of the tank and becomes super-heated, while non-uniformtemperature strata are formed in the lower portion of the tank. To makematters worse, the heat accumulation at the top of the tank tends torapidly dissipate because the insulation 13 in the tank cannoteffectively retain the high energy heat from the super heated water.With sequential pulse or burst heating of water as described herein,water in the tank 11 is more uniformly heated. This reduces theoccurrence of hot or cold spots in the strata from the top to the bottomof the tank. The creation of super heated water is also reduced andefficiency is increased.

[0066] The sequencing described above may also be combined withcontrolled introduction of cold water through an outlet or conduit 155of a mixing valve 157 positioned in the dip tube 14. The valve 157 maybe controlled through a communication link V_(I/O) coupled to thecontrol circuitry in box 17′ or, alternatively (and not shown), thecircuitry in box 17 when it is configured to control multiple heatingelements. Thus, for example, if super heating is sensed by the sensor18′ in the upper portion of the tank, an amount of cold water may beintroduced into the top portion of the tank 11 through the outlet 155 tolower the temperature of the heated water.

[0067] Yet another water heater 160 embodying the invention is shown inFIG. 6. The water heater 160 shares many common elements with the waterheaters 10 and 150, and common elements are designated with the samereference numerals in FIGS. 1, 5 and 6. For the embodiment shown in FIG.6, the water tank 160 defines a volume 165 having an approximately uppertwo-thirds volume 170 and an approximately lower one-third volume 175.The inlet opening 22 is disposed in the lower one-third volume 175 andintroduces cold water into the tank 11. The outlet opening 24 isdisposed within the upper two-thirds volume 170.

[0068] As shown in FIG. 6, both heating elements 16 and 16′ extend intothe lower one-third volume 175 of the tank 11. The heating elements 16and 16′ are controlled by control circuitry stored in control box 17which receives input from temperature sensors 18 and 18′. Alternatively,the water heater 160 may include more than one control box, may includemore than two heating elements and may include more than two temperaturesensors.

[0069] Similar to what was disclosed for water heater 150, the heatingelements 16 and 16′ are activated sequentially or at some predeterminedfrequency or fashion so that heat is transferred to the tank 11 in abalanced or uniform manner. Additionally, heating elements 16 and 16′are preferably activated by controller 17 utilizing proportion bandcontrol techniques.

[0070] In the preferred embodiment of water heater 160, the heatingelements 16 and 16′ are arranged in a plane 180 substantially orthogonalto the longitudinal axis 185 of the tank 11 (i.e., in a substantially“horizontal” plane) (see FIG. 7). However, the heating elements 16 and16′ may be place in any other configuration in the approximately lowerone-third volume 175 as long as both elements are in the approximatelylower one-third volume 175 (See FIG. 6). Also, if additional heatingelements are used, they too are located in the approximately lowerone-third volume 210.

[0071] Typically, water heaters of the prior art rarely utilize theupper heating element. The upper heating element is typically activeonly when the water heater is first installed, when the water heater hasnot been used for a long period of time, or when a large amount of hotwater has been extracted from the tank in a short period of time. Exceptfor these rare occurrences, the upper heating element of the prior artis rarely used. Thus, most of the water heated over the life of the unitis heated using only the lower element. The use of only the lowerelement is energy inefficient, requires a large period of time forrecovery of the water temperature to set point temperatures, and oftenrequires a large reserve storage tank of heated water to insure that anadequate supply of hot water is present when needed. The water heater160 overcomes the above-described deficiencies by placing the secondheating element 16′0 in the approximately lower one-third volume 175 ofthe tank 11. Arranging the elements 16 and 16′ this way and controllingthe operation of the elements 16 and 16′ by generating sequential pulseshaving proportional band control allows the water heater 160 to utilizemore efficient water heating strategies. This results in the elements 16and 16′ having an improved effective transfer of heat energy to thewater. Furthermore, elements 16 and 16′ more evenly distribute wattdensities, which reduces vaporization losses. Consequently, the waterheater 160 has a faster recovery time while using less energy thanconventional heaters of the prior art. Moreover, the water heater 160can have a more compact tank size for comparable hot water demands thanthe prior art.

[0072]FIG. 8 illustrates another water heater 200 embodying theinvention. Water heater 200 includes a permanently enclosed water tank205, a shell 210 surrounding water tank 205, and foam insulation 210filling the annular space between the water tank 205 and the shell 210.The water tank 205 has an outer surface 206. Water inlet line or diptube 215 and water outlet line 220 enter the top of water tank 205. Thewater inlet line 215 has an inlet opening 225 for adding cold water nearthe bottom of water tank 205. Water outlet line 220 has an outletopening 230 for withdrawing hot water from near the top of water tank205.

[0073] The water heater 200 further includes a first resistance heatingelement 235 and a second resistance heating element 240 extendingthrough the wall of the water tank 205. It is envisioned that theheating elements 235 and 240 may be placed anywhere within the tank 205and may be of any particular shape. However, preferably, the first andsecond heating elements 235 and 240 are in a lower one-third volume ofthe tank 200, and are in a plane substantially orthogonal to alongitudinal axis (similar to FIG. 7). In addition, although theinvention will be described with two heating elements 235 and 240, thewater heater 200 may include additional heating elements or may containjust one heating element 235. For example, a commercial tank waterheater (as compared to a residential tank water heater) may contain asmany as fifteen heating elements.

[0074] The water heater 200 includes a first water temperature sensor245 and a second water temperature sensor 250. Both water temperaturessensors 245 and 250 are mounted on the outer surface 206 of water tank205. The water temperature sensors 245 and 250 are preferablythermistors and are thermodynamically coupled to the water in the watertank 205. Preferably, the water temperature sensor 250 is located on alower half of the tank 205 and the temperature sensor 245 is located onan upper half of the tank 205. However, it is envisioned that the watertemperature sensors 245 and 250 may be mounted on the same half of thetank 205. Additionally, the water heater 200 may include additionaltemperature sensors or may contain only one temperature sensor 245.

[0075] The water heater 200 may include an ambient or room temperaturesensor 255. The ambient temperature sensor 255 is located external tothe water heater 200, but is located within the surrounding environmentof the water heater 200 and senses the temperature of the surroundingenvironment of the water heater 200. Of course, the water heater 200 mayinclude additional ambient temperature sensors and may include othersensors (e.g., a water consistency sensor).

[0076] The water heater 200 includes a proportional band controller orcontrol unit 260 electrically connected to the first and second heatingelements 235 and 240, the first and second water temperature sensors 245and 250, and ambient temperature sensor 255. In general terms, thecontroller 260 receives a two-hundred-forty volt alternating current(AC) signal from power line 265; modulates a first and secondproportional band signal provided to the first and second heatingelements 245 and 250, respectively; receives a first and second watertemperature signals from the first and second temperature sensors 245and 250; and receives an ambient temperature signal from ambient sensor255.

[0077] As shown in FIG. 9, the controller 260 includes a housing 267having a visual display area 270 and a user entry area 275. The visualdisplay area 270 includes a plurality of light-emitting diodes (LEDs).The LEDs include a first element LED2, a second element LED3, a systemLED4, a heat LED5, an alert LED6 and a power LED7. Power LED7 ispreferably a red LED and lights any time the electronics are active(i.e., “on”). System Led4 is preferably green and is used to indicatethe overall status of the system. During normal operation, the systemLed4 blinks approximately one blink per second. The fact that the systemLed4 is blinking regularly indicates that the water heater is workingproperly. Heat Led5 blinks in unison with the system Led4 when thecontroller 260 is in a “heating” mode (i.e., the water heater is heatingthe water to a desired). First element Led2 and second element Led3activate whenever the respective heating elements are active. Alert Led6and heating Led5 are in the same package. Alert Led6 works inconjunction with the system Led4 to indicate the status of the waterheater 200.

[0078] During normal operation, if the controller 260 is in a “Stand-by”mode (i.e., the temperature of the water is equal to or greater than thedesired water temperature), only the system Led4 blinks. If thecontroller 260 is in the heating mode, the controller 260 blinks thesystem Led4 and the heating Led5 in unison. If for any reason there isan error state, then the heating Led5 changes to the Alert Led6, whichis red. During the error state, the system Led4 blinks an error codeindicating the type of error. Of course, other LEDs can be added, andany of the disclosed LEDs can be removed or modified. Additionally, anaudible speaker can be included to provide audible indication, or theinformation provided by the LEDs can be communicated by other visualindicators (e.g., a liquid crystal display).

[0079] The user entry area 280 includes an entry dial 283 for a user toenter a desired water temperature. The entry dial 283 includes an offposition (i.e., the water heater 200 is “off”), a vacation position, anda plurality of positions between a low or cold water temperature and ahigh or hot water temperature. If the entry dial 285 is in the vacationposition, then the controller is in a “vacation” mode. The “vacation”mode heats the water to a preset temperature lower than the normaltemperature range of the water heater. Alternatively, the user entryarea 275 may include other possible devices for entering a desired watertemperature state including a plurality of push buttons with a digitalLCD display. Of course, the visual display area 275 and the user entryarea 280 may be mounted in a second control box located remotely fromthe water heater 20 (i.e., not mounted on the water heater 20). Thesecond control box in communication with the controller 260 eitherthrough a hard-wired connection, or through RF or other appropriatecommunication scheme.

[0080] The controller 260 includes a control circuit 285, which isschematically represented in FIG. 10. In general terms, the controlcircuit 285 includes a power supply 290, a zero crossing detector 295, alow-voltage reset circuit 300, a temperature sensing circuit 305, athermostat circuit 310, an LED control circuit 312, a microcontrollerU1, a memory unit 315, a first driving circuit 320, a second drivingcircuit 325, and a dry fire circuit 330.

[0081] As shown in FIGS. 10, the power supply 290 receives ahigh-voltage AC signal (e.g., AcIn=240 VAC) from line 260 (FIG. 8) andcreates a low voltage AC signal (e.g., AcOut=9 VAC), an unregulateddirect current (DC) signal (e.g., V-SNS=5 VDC), and a regulateddirect-current signal (e.g., Vcc=5 VDC). An exemplary power supply 290is shown in greater detail in FIG. 11.

[0082] As shown in FIG. 11, the power supply 290 includes a transformerT2 having a primary coil and a secondary coil for transforming thehigh-voltage AC signal (AcIn) to the low-voltage AC signal (AcOut). Theresulting low-voltage AC signal (AcOut) is provided to the zero-crossingdetector 295 (FIG. 10) and to a switch S1, which is a single-throw,single pole (SPST) switch connected to the high side of the secondarycoil. When the switch S1 is closed, the control circuit 285 is active.

[0083] The power supply further includes a full-wave bridge rectifierD8, a capacitor C26, a zener diode D9, a voltage regulator U9, andcapacitors CU1, CU2, CU4, CU7 and CU8. The bridge rectifier D8 rectifiesthe low-voltage AC signal (AcOut) and the capacitor C26 filters thesignal resulting in the unregulated DC signal (VSNS). The zener diode D9caps the unregulated DC signal (VSNS) and protects the input of thevoltage regulator U9 from short-term, over-voltage transients. Thevoltage regulator U9 regulates the voltage to a Vcc signal of five voltsand each of the capacitors CU1, CU2, CU4, CU7 and CU8 on voltageregulator U9 are decoupling capacitors dedicated to a respectiveintegrated circuit. For example, capacitor CU1 is a decoupling capacitorfor integrated circuit U1.

[0084] Referring back to FIG. 10, the power supply 290 provides the lowvoltage AC signal (AcOut) to zero-crossing detector 295. An exemplaryzero-crossing detector 295 is shown in greater detail in FIG. 12.Zero-crossing detector 295 provides an output signal (ZeroCross) whichindicates each time the detector 295 detects that the low voltage signal(AcOut) has changed plurality. The zero-crossing detector 295 includesresistors R55, R61 and R53, capacitor C21, diode D1, and transistor Q8.The resistor R55 receives the low-voltage AC signal (AcOut). The diodeD1, capacitor C21, and resistor R61 are connected in parallel with oneend connected to resistor R55 and the base of transistor Q8 and theother end connected to the emitter of transistor Q8. Resistor R53 hasone end connected to Vcc and the other end connected to the collector oftransistor Q8. The zero-crossing signal (ZeroCross) is generated at thecollector of transistor Q8. As the AC voltage changes polarity, Q8 goesback and forth between the off state and saturation, generating a seriesof pulses having a front edge. The front edge of each pulse correspondsto a zero crossing.

[0085] Referring back to FIG. 10, the control circuit 285 includes alow-voltage reset circuit 300. An exemplary low-voltage reset circuit300 is shown in greater detail in FIG. 13. The low voltage reset circuitincludes an integrated circuit U3, which is preferably a MotorolaMC34064P-5 (although other circuits may be used) connected to acapacitor C18, and resistors R45 and R46. The integrated circuit U3provides an under voltage reset protection signal to the microcontrollerU1. In the event that power should fail or “brown” out, integratedcircuit U3 causes the microcontroller U1 to reset. Preferably, thisoccurs as soon as the requested DC signal drops below four and one-halfvolts. The low-voltage reset circuit ensures that the control circuit285 safely operates and does not malfunction due to low-line power.

[0086] Referring back to FIG. 10, the control circuit 285 includes atemperature sensing circuit 305. The temperature sensing circuit 305 incombination with first and second water temperature sensors 245 and 250transmits a water temperature for the water heater 200 to themicrocontroller. As shown in greater detail in FIG. 14, the temperaturesensing circuit includes resistors R70 and R71, and thermistors RT1 andRT2, which have a negative temperature coefficient. Resistor R70 andthermistor RT1 form a first voltage divider resulting in a firsttemperature signal (First-Sensor), and resistor R71 and thermistor RT2form a second voltage divider resulting in a second temperature signal(Second-Sensor). Since the first and second voltage dividers arepreferably the same, only the first voltage divider will be discussed indetail. As the temperature on the outside of the water tank 205increases, the resistance in the thermistor RT1 decreases causing theoutput voltage (First-Sensor) to increase. The voltage (First-Sensor) isread by an analog-to-digital (A/D) converter in microcontroller U1resulting in an eight-bit number. The eight-bit number is used as anindex to a lookup table that has a plurality of corresponding sensedtemperatures. Based on the eight-bit number, a sensed temperatureresults.

[0087] As the water inside the tank 205 increases in temperature, thereis an increasing error in what the temperature sensor 245 or 250 senses.That is, the thermal conductive path from the water through the materialof the water tank 205 has a lag time differential. To correct this, thesensed temperature value read from the lookup table is “corrected” by alinear equation. The corrected temperature is used in making waterheating decisions by the microcontroller U1.

[0088] Referring back to FIG. 10, the control circuit includes athermostat 310. As shown in greater detail in FIG. 15, the thermostat isa potentiometer R65 wired as a voltage divider and having a resistancerange (e.g., 20 kOhms). The output signal of the voltage divider(Thermostat) is converted to an eight-bit number by the microcontrollerU1 and then scaled to produce a set-point temperature value. Theset-point temperature value is the temperature to which the water willbe heated.

[0089] Referring back to FIG. 10, the control circuit 285 includes anLED control circuit 312. The LED control circuit 312 controls theactivation of the light-emitting diodes Led2, Led3, Led4, Led5, Led6 andLed7. As shown in greater detail in FIG. 16(a), the LED controller 312includes resistors R56, R57, R58, R59, R60, R47, R48, R49, R50, R51 andR52, and transistors Q3, Q4, Q5, Q6 and Q7. When switch S1 (FIG. 11) isclosed, the power supply 290 generates a regulated low-voltage DC signal(Vcc) that is provided to Led7 and resistor R52. The providedlow-voltage regulated DC signal (Vcc) lights Led7. For controlling Led2,Led3, Led4, Led5 and Led6, a five-bit signal is provided to resistorsR56, R57, R58, R59 and R60. If any of the bits are high, a low-voltageDC signal is provided to the respective resistor R56, R57, R58, R59 orR60 resulting in a base current sufficient to allow current flow throughthe respective transistor Q3, Q4, Q5, Q6 or Q7. The current flows fromVcc through the transistor Q3, Q4, Q5, Q6 or Q7, through the respectivelight emitting diode Led2, Led3, Led4, Led5 or Led6, to ground.

[0090] Referring back to FIG. 10, the control circuit includes amicrocontroller or processor U1 and a memory unit 315. Themicrocontroller U1, which is also shown in FIG. 16(a), is preferably a28-pin Motorola MC68HC705P6A (although other microcontrollers may beused). The microcontroller U1 includes an eight-bit input/output port(pins 3-10), a three-bit serial interface (pins 11-13), a four-bitanalog to digital converter (pins 15-19), memory for storing a softwareprogram that operates the microcontroller, and two pins (pins 26 and 27)for receiving a signal from an oscillator 317 (FIG. 17). The memory unit315 includes a two hundred fifty six byte Electrically ErasableProgrammable Read Only Memory (EEPROM) chip U4. The EEPROM U4 is used tostore configuration data, such as water heater construction specifics(e.g., operating voltage, tank water capacity, resistances of variouselements, etc.), user usage pattern data, element type data, and otherrelated data. With the EEPROM data and real-time sensory data (e.g., thesensed temperature of the first and second water temperature sensors 245and 250), the micro controller U1 implements a software program tocontrol the heating elements to heat and maintain water temperature. Inaddition, the software program includes at least one subroutine todetermine whether water is surrounding each heating element.

[0091] Referring back to FIG. 10, the control circuit includes a firstdriving circuit 320 and a second driving circuit 325 that control thepower being provided to the first and second heating elements 235 and240, respectively. The driving circuits are identical and, thus, onlydriving circuit 320 will be discussed in detail. As shown in FIG. 16(b),the first driving circuit 320 includes resistors R66 and R86 a triac Q1,and an opto coupled zero-cross triac driver U5. The triac driver U5 isgate driven as determined by gate pulses being received from the outputof the microcontroller U1. A pulse train is generated by themicrocontroller U1, which determines the power levels being delivered tothe heating element 235 (FIG. 10). For example, the microcontroller U1may provide a pulse train to the triac driver U5 resulting in asixty-six percent power transfer (i.e., sixty-six percent of theavailable power is transferred to the heating element), or may provide apulse train to the triac driver U5 resulting in a forty percent powertransfer. The triac driver U5 is coupled to the zero-crossing detector295 to insure that the triac turns completely off when the set pointtemperature is reached. Without the use of driver U5, the triac Q1 couldremain partially open in a conduction state and potentially effect thereliability of the control circuit 285.

[0092] Referring back to FIG. 10, the control circuit includes a dryfire circuit 330. As shown in greater detail in FIGS. 16(a) and 16(b),the dry fire circuit 330 includes data latch U2 (16(a)), a firstresistor ladder 335 (16(a)), a second resistor ladder 340 (16(a)), avoltage sensing amplifier 345 (16(b)), a current sensing amplifier 350(16(b)), resistors R90, R91, R92, R97, R98 and R100 (all in 16(b)),transistors Q9 and Q10 (both in 16(b)), a current sensor T1 (16(b)), anda resistor R44 (16(b)). The data latch U2 is preferably a Motorola74HC374 data latch (other data latches may be used) and is used to holda five-bit data word that controls the first and second resistor ladders335 and 340. The first resistor ladder 335 generates a voltage that isused as a reference by the voltage sensing amplifier 345. Once thisreference voltage has been set or calibrated, the data latch U2 is usedto control the second resistor ladder 340 to generate a voltage that isused as a reference by the current sensing amplifier 350. The latch alsoholds three additional data bits. The first data bit (bit 7), controlsone of the display LEDs; the second data bit (bit 6), selects theEEPROM; and the third data bit (bit 5), enables communication withoff-board testing equipment. The current sensor T1 and the resistor R44create a voltage that is proportional to the current being provided tothe heating elements. Transistors Q9 and Q10 select which amplifier iscurrently providing a signal to the microcontroller U1.

[0093] The basis for the “DryFire” test is the measurement of the peakvoltage and peak current on an “almost” cycle by cycle basis. The reasonthat the measurement is not exactly cycle-by-cycle is that the voltageis measured after it has been rectified and filtered. Changes in the ACline voltage manifest as changes in the rectified DC voltage. Because ofthe time constant of the capacitor C26, with the resistance in thesecondary windings of the power transformer, voltage and current samplesare taken on a cycle-by-cycle basis and stored in a buffer. When thebuffer is full, the voltage samples are examined to determine whetherthe voltage was stable during the time period it took to fill thebuffers. If the variance is within acceptable limits, the voltage andcurrent samples are average and a simple resistance calculation isperformed (i.e., R=V/I).

[0094] When the manufacturer assembles the water heater 200, themanufacturer programs into the memory unit 315 the components used forassembly of the water heater 200, the capacity of the water tank 205,and/or product information about particular components of the waterheater 200. For example, the manufacturer may program one or more tankcharacteristics and/or one or more element characteristics into thememory unit. The tank characteristics may include, but are not limitedto, tank diameter, tank height, tank storage capacity, etc. The tankcharacteristics determine heating convection current flow patternswithin the tank 205 that create different temperature water stratalayers in the tank 205. The element characteristics may include, but arenot limited to, number of elements, element type, voltage of an element,physical location of an element (e.g., upper and lower, orside-by-side), element watt density, etc. The element characteristicshelp to provide information on how effectively the elements 235 and 240will heat the water.

[0095] In addition, some of the tank or element characteristics can bedetermined by the microcontroller U1. For example, the microcontrollercan calculate an element wattage for a particular element by applying avoltage to the element and calculating a resistance for the element overtime.

[0096] Preferably, all of the water heater tank characteristics andelement characteristics are programmed into the memory unit 315. Basedon the variables and characteristics, the microcontroller U1 obtainsfrom a lookup table a code specific to the water heater 200. Thesoftware of the microcontroller U1 creates a heating strategy for thewater heater 200 based in part on the water heater code (discussedbelow). The microcontroller U1 can update the water heater code if itsenses that an element has been replaced or if a repairperson reprogramsthe data stored in the memory unit 315. Additionally, although themanufacturer programs each variable or characteristic into the memoryunit 315, it is envisioned that the manufacturer can directly programthe code into the memory unit 315.

[0097] Because there are a diversity of tank characteristics andelements used in the manufacture and construction of electric waterheaters, one heating strategy alone is unable to account for thenumerous constructions. Instead, the software assigns a code to thewater heater 200 based on the variables and characteristics of the waterheater 200. The variables and characteristics define a water heatersignature and, when used with a water heater usage pattern, create amore reliable effective and energy efficient water heater.

[0098] In operation of water heater 200 and referring now to FIG. 18, auser “turns-on” the water heater 200 (act 500) by turning the thermostat310 clockwise from the off position. This closes switch S1. Upon closingswitch S1, the power supply 290 generates the low-voltage AC signal(AcOut), the unrectified DC signal (V-SNS) and the rectified DC signal(Vcc). Once the power source generates a Vcc greater than four andone-half volts, the low voltage reset 300 brings the microcontroller U1out of reset. If at any time the voltage drops below four and one-halfvolts (e.g., a user turns the system off, a “black-out” occurs, or a“brown-out” occurs), the low voltage reset 300 provides a signal to themicrocontroller U1 resetting the microcontroller U1.

[0099] At act 505, after the microcontroller U1 comes out of reset, thesoftware initializes the microcontroller U1. The software resets allvariables to their default values, and resets all outputs to theirrespective default states.

[0100] At act 510, the microcontroller performs a “DryFire” test. Theterm “DryFire” refers to the heating of a heating element 235 or 240that is not submerged in water. Usually, a “Dry Fire” will destroy orburn-out the heating element 235 or 240 in less than a minute. Thecontrol circuit 285 performs the “DryFire” test to determine whether theheating element is surrounded by water.

[0101] In general terms, the control circuit 285 performs the “DryFire”test by measuring the peak current and the peak voltage being applied toeach heating element 235 and 240 and making a resistance calculationbased on the measurement. For example, by applying a voltage to one ofthe heating elements 235 or 240 for a specific period of time andmeasuring the resistance at the beginning and end of the test period,the status of the heating element 235 or 240 can be determined. As theelement 235 or 240 heats up, its resistance increases. If the element isin water, the element reaches equilibrium (i.e., a steady temperatureand resistance), very quickly. Conversely, if the element 235 or 240 is“dry”, it continues heating and reaches high temperatures (andresistances) in a very short time. At the end of the test, the beginningand ending resistances are compared. For a “wet” element, the differencebetween the beginning and ending resistances is small, while for a “dry”element, the difference between the beginning and ending resistances ismany times larger than when the element is wet.

[0102] In addition, by varying the length of the DryFire test, the wattdensity of the heating element 235 or 240 can be accurately measured.Based on the watt density, the microcontroller U1 can update the waterheater code.

[0103] An exemplary method for performing the DryFire test is shown inFIG. 19. At act 605, the microcontroller U1 deactivates all the LEDsduring the DryFire test. Deactivating the LEDs ensures that the blinkingof the LEDs does not affect the test. At act 610, the software sets anelement number indicating the first heating element 235 is being tested.At act 615, the software sets the operating mode for the microcontrollerU1 to a DryFire mode which informs all subroutines that themicrocontroller U1 is performing a DryFire test. At act 620, thesoftware clears all DryFire error flags. The DryFire error flagsindicate whether the most recent DryFire test (if one occurred) resultedin an error. For example, if the previous DryFire test resulted in anerror flag corresponding to the first element being “dry”, then themicrocontroller U1 resets the error flag pending the results of thecurrent test.

[0104] At act 625, the microcontroller U1 calibrates the voltageamplifier 345. Before any voltage samples can be taken for DryFirecalculations, the voltage amplifier 345 must be calibrated using avariable reference voltage generated by data latch U2 and resistorladder 335. To accomplish this calibration, the microcontroller U1 firstselects the output of the voltage sensing circuit by driving Q10 intosaturation (Q9 is off). The reference voltage (V-REF) is then set to itshighest value. Next, the reference voltage (V-REF) is incrementallyreduced until the output of the voltage amplifier (Dry-Out) reaches apredetermined value. The reference voltage is then left at this value.

[0105] For example, V-SNS is a non-regulated DC signal having asteady-state component and a small “alternating current” component. Anyincreases or decreases in the signal being provided to the transformer(AcIn) will reflect in the small “AC” component of V-SNS signal. Inorder for the microcontroller U1 to notice any changes of significance,the voltage amplifier 345 amplifies small “AC” component changes. If,for example, the steady state is 2.0 volts, any reference voltage(V-REF) feeding resistor R88 (FIG. 16(b)) above 2.0 volts will result inno amplification taking place and the output of the amplifier will bezero. If the reference voltage (V-REF) is below 2.0 volts, amplificationwill take place. The reference voltage (V-REF) is adjusted so the outputof U7B is somewhere in the middle of its output swing (e.g., 0-3.5volts). The microcontroller U1 continues to reduce the reference voltage(V-REF) in steps until a desired output is reached (e.g., referencevoltage is equal to 1.5 volts). Thus, any changes in the line voltageare exaggerated by a factor equal to the gain of U7B.

[0106] At act 630, microcontroller U1 calibrates the current amplifier350. As with the voltage amplifier 345, the second stage, U8B (FIG.16(b)), must be calibrated before sampling can begin. The currentsensing circuit is selected by driving Q9 into saturation (Q10 is off)and then incrementally adjusting the reference voltage (I-REF) similarto the reference voltage (V-REF).

[0107] At act 635, the software determines whether the voltage andcurrent amplifiers 345 and 350 were properly calibrated. If there was anerror in the calibration, then the software sets a calibration errorflag(s) (act 640) to a positive result and proceeds to act 660. If thecalibration did not result in any errors, then the microcontroller U1proceeds to act 645.

[0108] At act 645, the microcontroller U1 performs a DryFire test forthe first element 235. For the test, instantaneous voltages and currentsare measured at their peak values. This is accomplished by sampling thesignal from the voltage and current amplifying circuits 345 and 350(Dry-Out) relative to a zero crossing of the low-voltage AC signal(AcOut). At the appropriate zero crossing, a timer is started for eachof the amplifying circuits 340 and 350. A time-out variable is used totake the voltage or current samples at a predetermined time period withrespect to the zero crossing when the voltage and current waveforms areat their peak. The instantaneous voltage and current samples are eachloaded into separate buffers within the microcontroller U1. When thebuffers are full, the data is analyzed to determine if the line voltagehas been stable during the sampling period. If the sampled voltage isstable, an average voltage and current is computed, and a resistancecalculation is made. Calculations continue in this manner for theduration of the DryFire test. At the end of the test, the beginning andending resistance values are subtracted to find out how much theresistance has changed over the course of the test. The basis of thetest is not the actual value of resistance (which is different for eachtype of heating element), but the difference in resistances from thebeginning of the test to the end of the test.

[0109] At act 650, the microcontroller U1 determines whether the firstelement 235 is dry. If the calculated resistance difference is greaterthan a set resistance change value (which may vary depending upon theheating element used) then the microcontroller U1 determines that theelement is not surrounded by water (i.e., “dry”) and proceeds to act655. If the microcontroller U1 determines that the calculated resistancechange is equal to or less than a set resistance change value, then themicrocontroller U1 determines that the element is surrounded by waterand proceeds to act 660.

[0110] At act 655, the software sets a first element error flag to apositive result. A positive first element error flag informs subsequentsubroutines that the first element 235 is not surrounded by water.Consequently, later subroutines will not use this element to heat thewater. The microcontroller U1 will also set a ReCheck timer to 180minutes. The ReCheck timer will decrease in time until it reaches zerominutes. When the ReCheck timer reaches zero, the microcontroller U1will perform another DryFire test on that element.

[0111] At act 660, the microcontroller U1 sets the element number to thesecond element. At act 665, the microcontroller U1 repeats acts 625,630, 635, 640, 645, 650 and 655 for the second element to determinewhether the second element is dry. If the microcontroller U2 determinesthe second element is dry, it will set a second element error flag to apositive result. Of course, if the water heater includes more than twoheating elements, then the microcontroller U2 performs a dry test forthe remaining elements. Additionally, if the water heater contains onlyone heating element, then the microcontroller U2 will not perform acts660 or 665.

[0112] Referring back to FIG. 18, at act 515, the software determineswhether a “ReCheck” timeout is greater than zero. The ReCheck timeout isa timer (e.g., twenty milliseconds) used by the software to inform thesoftware when to sample the temperature sensors 245, 250 and 255, andcreate or modify a heating strategy for heating the water containedwithin the water heater 200. If the ReCheck timeout is greater thanzero, then the software proceeds to act 520. If the ReCheck timeout isless than or equal to zero, then the software proceeds to act 525.

[0113] At act 520, the microcontroller U1 “blinks” the system Led4, theheat Led5 and the alert Led6. That is, the software performs asubroutine that activates appropriate LEDs depending on the mode thesoftware is in or if an error flag has occurred. For example, duringnormal operations, microcontroller 305 generates a signal resulting inthe system Led4 to blink on and off. If the software is in a heatingmode (discussed below), then the heat Led5 blinks in unison with thesystem Led4. If the software has a positive error flag, the alert Led6works in conjunction with the system Led4 to indicate the status of thewater heater 200 to an operator or repairperson.

[0114] If the ReCheck timeout is less than or equal to zero, then themicrocontroller U1 proceeds to Act 525. In general terms, themicrocontroller U1 samples temperature sensor samples (act 525),computes a water temperature (act 530), computes the thermostat setting(act 535), establishes an operating mode (act 540), sets a heating cyclestate (act 545), and sets a heating priority (act 550). An exemplarymethod implementing acts 525, 530, 535, 540, 545 and 550 is shown inFIG. 18. In addition, the microcontroller U1 stores data for creating ausage history (act 555) and blinks the LEDs (560).

[0115] At act 705 (FIG. 20(a)), the microcontroller U1 samplestemperature sensor 245 and loads a resulting first voltage into thesoftware for processing. At act 710, the microcontroller U1 samplestemperature sensor 250 and loads a resulting second voltage into thesoftware for processing. At act 715, the microcontroller U1 converts thefirst and second sampled voltages to a first and second sensedtemperatures, respectively, using a temperature lookup table. Thelook-up table contains a plurality of voltage ranges having a respectiveassociated temperature. For example, if the first temperatures sensorgenerates a 2.1 volt signal, the associated temperature may be 110degrees fahrenheit. The look-up table can vary depending on the sensorused. After obtaining the first and second sensed temperatures, thesoftware modifies the sensed temperatures to take into account any lagtime in obtaining the temperature. That is, as the water inside the tank205 increases in temperature, there is an increasing error in what thetemperature sensor 245 or 250 senses. The thermal conductive path fromthe water through the material of the water tank 205 has a lag timedifferential. To correct this, the temperature values read from thelookup table are “corrected” for the lag. The corrected first and secondtemperatures are used in making water heating decisions by the software.

[0116] At act 720, the microcontroller U1 loads or samples a signal fromthe thermostat 310. If the microcontroller U1 determines that thethermostat voltage corresponds to the thermostat being in off position(act 725), then the software sets an operating mode equal to an offstate (act 730) and returns to act 555 of FIG. 18. For example, if thethermostat voltage is less than 0.1 volts, then the software determinesthe thermostat is in an off position and turns off the controller 260.If the thermostat voltage is greater than a voltage corresponding to anoff position (act 725), then the software proceeds to act 735.

[0117] At act 735, the software determines whether the operating modewas previously set to off (i.e., the system was just turned on). If theoperating mode was previously off, then the software changes theoperating mode to “stand-by” (act 740). As will be discussed in moredetail below, when the water heater 200 is in a stand-by mode, thecontroller 260 is not increasing the temperature of the water. If theoperating mode is in a mode other than the off operating mode, then thesoftware proceeds to act 745.

[0118] At act 745, the software compares the thermostat voltage with aset voltage representing the vacation position of the thermostat. Forexample, if the thermostat voltage is less than 0.7 volts, then thesoftware determines that the thermostat is set to the vacation positionand proceeds to act 750. If the thermostat voltage is greater than 0.7volts, then the software determines that a user has set the water heaterto a desired temperature and proceeds to act 755.

[0119] At act 750, the software sets the set point temperature equal toa vacation temperature (e.g., 90 degrees Fahrenheit). The vacationtemperature may be a manufacturer-determined value, or may be preset bya user. After setting the set-point temperature, the software proceedsto act 760 (FIG. 20(b)).

[0120] At act 755 (FIG. 20(b), the software computes a set pointtemperature based on the sampled thermostat voltage. The microcontrollerU1 preferably uses a second lookup table, but may alternatively use aformula based on the input voltage.

[0121] At act 760, the software computes a heater-on temperature. Theheater-on temperature is the temperature at which one or more elementsreceive a power signal. The heater-on temperature is the set-pointtemperature minus a hysteresis temperature. The hysterisis temperatureis the number of degrees fahrenheit (e.g., 10 degrees fahrenheit) thatthe water temperature drops below the set-point temperature beforeheating occurs. Thus, by calculating a heater-on temperature, themicrocontroller U1 avoids “under cycling”.

[0122] At act 765, the software determines whether the operating mode isin a “stand-by” mode or a “heating” mode. If the operating mode is setto stand-by, the software proceeds to act 770. If the operating mode isset to heating, then the software proceeds to act 775.

[0123] At act 770, the software determines whether the lower-tanktemperature (from temperature sensor 250) is less than or equal to theheater-on temperature. If the lower-tank temperature is less than orequal to the heater-on temperature, then the software determines thatthe water should be heated and proceeds to act 780. If the lower-tanktemperature is greater than the heater-on temperature, then the softwaredetermines that the water should not be heated and proceeds to act 800.

[0124] At act 780, the software sets the operating mode to the heatingmade indicating that the water should be heated. After setting theoperating mode to heating, the software resets all operating statevariables and timeouts for another heating cycle (act 785). For example,the software resets the ReCheck timeout (e.g., to twenty milliseconds.)

[0125] If, at act 765, the software determines the operating mode is setto heating, the software proceeds to act 775. At act 775, the softwaredetermines whether the lower tank temperature is greater than or equalto the set point temperature. If the lower tank temperature is greaterthan or equal to the set point temperature, then the software determinesthat the water should continue to be heated, and therefore stays in theheating mode and proceeds to act 800. If the lower tank temperature isless then the set point temperature, than the software determines thatthe water has been properly heated and proceeds to act 785.

[0126] At act 785, the software changes the operating mode to stand-by(i.e., indicating that the water temperature no longer should increase).At act 790, the software determines whether the first heating element235 is surrounded by water (this is assuming the first element is abovethe second 235). If the first heating element 235 is not surrounded bywater (i.e., the element is dry), then the software sets the ReChecktimeout variable to two minutes (act 795). By changing the length of theReCheck timeout variable, the software allows the water tank to fillwith more water before heating with the first element. Of course, theamount of time the software sets the ReCheck timeout variable to canvary, and a specific value is not required for purposes of the inventionto work. If the first element does have water surrounding the element(i.e., a wet state has resulted), then the software proceeds to act 800.

[0127] At act 800 (see FIG. 20(c)), the software determines whether atemperature slope calculation period has elapsed. If the period haselapsed, then the software resets the timer and computes a temperatureslope (act 805). Computing the temperature slope allows thedetermination of whether a water draw is occurring. At regular intervals(e.g., 90 seconds), the most recent temperature sample of the tank iscompared with previous samples stored in the memory unit (315). Based onthe temperature values, a temperature slope or rate of change oftemperature is calculated for the water. If the user is drawing water, alarge negative slope value will result informing the software that adraw of water is in progress.

[0128] At act 810, the software sets a duty cycle that determines theamount of power to be transferred to each heating element. The amount ofpower varies depending on the temperature of the water and the waterheater code for the water heater 200. In addition, the amount of powermay take into account a water heater usage pattern (which is stored inthe memory unit 315), the ambient temperature, a water consistencyvalue, or other information.

[0129] For act 810, the software obtains from the memory unit 315 thewater heater code and past records of data stored by the water heater.The past records are stored each time the software completes act 555(FIG. 18), and each record includes the time of day, duration of pastheatings, rate of change (slope) in water temperature decline and rise,and may additionally include other information such as ambient roomtemperature. As the controller 260 heats the water, it looks into thememory unit 315 for recorded information of similar circumstances duringthe same time period in previous days and/or weeks. If it appears thatthe user is using about the same amount of water during any given periodthen the water will be heated at a standard rate for the water heatercode that will satisfy the anticipated consumption rate of heated water.If the stored data would indicate that there may be no further usageafter the present heating cycle, the water then will be heated veryslowly at a lower duty cycle to minimize energy consumption. If there isan abrupt and rapid decline (i.e., negative temperature slope) in watertemperature, the software will calculate a new duty cycle according tothe present usage condition of the water heater. As usage patternschange the old records will be modified to reflect the current operatingconditions. For the preferred embodiment, the base line formula inconsidering what minimum water temperature flow rates will be acceptableis a minimum recovery equal to ten gallons per hour at sixty degreeFahrenheit temperature rise.

[0130] With this formula, product code information and usage records,the power input ratios versus temperature rate change are used indetermining heating strategies. The strategies provide input powerlevels to meet or exceed the minimum recovery rate, while keeping energyefficiency to a maximum. As conditions change in usage patterns thestrategy is modified to maintain the minimum recovery standard.

[0131] For example, a standard heating strategy for a first water heatercode having a first element wattage will differ when compared to aheating strategy for a second water heater code having a second elementwattage. Two exemplary heating strategies for the second element 240 areshown in Tables 1 and 2. TABLE 1 Water Heating Strategy for a FirstHeater Code Power or Duty Cycle of the Water Temperature Second Element<115° F. 100%  115° F. to 120° F. 66% 120° F. to 125° F. 57% 125° F. to130° F. 50% 130° F. to 135° F. 40% 135° F. > 20%

[0132] TABLE 2 Water Heating Strategy for a Second Heater Code Power orDuty Cycle of the Water Temperature Second Element <115° F. 100%  115°F.to 120° F. 80% 120° F.to 125° F. 66% 125° F.to 130° F. 50% 130° F.to135° F. 40% 135° F. > 20%

[0133] For water heater 200, the duty cycle or power applied to theheating elements 235 or 240 is based at least in part on the sensedwater temperature and the water heater code. The concept of a heatingstrategy dependent on a water heater code is unlike the method ofheating water for water heaters 10 and 150. For water heaters 10 and150, the duty cycle or power applied to the heating elements 16 and/or16′ is based on the difference between the sensed water temperature andthe desired water temperature. However, it has been determined thatincreasing the power to an element submerged in water at a given watertemperature may not result in an optimum water temperature gain whencompared to the power input. For example, assuming all other conditionsare the same, it has been determined that more heat can be transferredfrom an element to water when the water is at a cooler temperature. Asthe water temperature increases, less power needs to be provided to theheating element 235 or 240 regardless of the difference between thesensed temperature and the desired temperature (i.e., the excess powerwill not result in an optimum transfer when compared to the powerinput). Therefore, the software does not need to take into account thedifference between the desired temperature and the sensed temperaturefor heating the water. But it is envisioned that under somecircumstances (e.g., the usage pattern changes resulting in the waterneeding to be heated as fast as possible without a concern forefficiency) that a heating strategy may want to include a differencemeasurement.

[0134] At act 815, the software determines the “draw down” state. Thedraw down state indicates whether a user is currently drawing water andat what rate the user is drawing the water. The draw down state has fourvalues: “tank is heating”, “draw-down-one”, “draw-down-two”, and“recovering”. If the draw down state is “tank-is-heating”, then thesoftware proceeds to act 820. If the draw down state is “draw-down-one”,then the software proceeds to act 825. If the draw down state is“recovering”, then the software proceeds to act 830. If the draw downstate is “draw-down-two”, then the software proceeds to act 835.

[0135] At act 820, the software determines whether the temperature slopeis less than or equal to a threshold for a draw down. For example, ifthe calculated temperature slope is less than ten degrees Fahrenheitthen the software determines a draw down is in progress and sets thedraw down state to “draw-down-one” (act 840). If the temperature slopeis greater than the draw down threshold then the software determines adraw is not in progress and proceeds to act 870.

[0136] If the draw down state is currently “draw-down-one”, then thewater heater had previously been in a draw down (i.e., a user is usinghot water). At act 825, the software determines whether the temperatureslope is positive. If the temperature slope is positive, then thesoftware determines that the water heater is recovering and sets thedraw down state to recovering (act 845). If the temperature slope isstill negative, then the software determines the water heater is stillin a draw down and proceeds to act 870.

[0137] If the draw down state is currently set to “recovering”, then thewater heater is recovering from a draw down. At act 830, the softwaredetermines whether there has been another draw down (i.e., thetemperature slope is less than or equal to the threshold for a drawdown). If there was another draw down, then the software sets the drawdown state to “draw-down-two” (act 850). If the software determines thewater heater is still recovering, the program proceeds to act 870.

[0138] At act 835, the software determines whether the lower tanktemperature is greater than or equal to a heater-on temperature. If thelower tank temperature is greater than or equal to a heater-ontemperature, then the software sets the draw down state to recoveringand resets the temperature slope. If the lower tank temperature is lessthan the heater-on temperature, then the microcontroller U1 sets theduty cycle to full power (act 760). Of course, other duty cycles can beused depending upon the particular water heater and environmentalcircumstances.

[0139] At act 870, the software determines the heating priority for thewater heater. If the heating priority is “fifty-fifty” (discussedbelow), then the software sets the duty cycle to full power (act 875)regardless of the water temperature. Of course, other duty cycles can beused depending upon the particular water heater and environmentalcircumstances. If the heating priority is not in the fifty-fifty mode,then the software proceeds to act 880 (FIG. 20(d)).

[0140] At act 880, the software selects a case based on the previouslydetermined heating priority. The heating priority is used fordetermining which elements are active. For example, if the first elementis an upper element and the second element is a lower element (similarto FIG. 5), then under certain conditions both elements may be used. Forthis arrangement, if both elements are being used, then the heatingpriority will be fifty-fifty. If only one element is used, then theheating priority is zero-one-hundred. Alternatively, if the elements arein a substantially horizontal plane, both elements may be used in afifty-fifty arrangement (vs. only one element being used) to heat thewater.

[0141] At act 885, the software determines if the upper tank temperaturehas fallen (i.e. the temperature slope of the upper element is less thanor equal to a threshold). If the upper tank temperature has fallen, thenthe software sets the heating priority to “fifty-fifty” (act 887),resulting in both elements heating the water. If the upper tanktemperature has not fallen, then the software proceeds to act 555 (FIG.16).

[0142] At act 890, the software determines whether the upper tanktemperature has recovered (i.e., the temperature slope of the upperelement is greater than a threshold). If the upper temperature tank hasrecovered, then the software sets the priority to “zero-one-hundred”(act 895), resulting in only the second element 240 heating the water.If the upper tank temperature has not recovered, then the softwareproceeds to act 555 (FIG. 16).

[0143] Every eight hundred microseconds, the software performs a timerinterrupt event. The timer interrupt is used as a time base for varioustimeouts (e.g., the “ReCheck” timeout). During each interrupt, themicrocontroller's timer is reset and the timeout variables are decreasedif their value is still greater than zero. Once a timeout value reacheszero, the associated routine can be performed at that time, or can beperformed during the main loop. As shown in FIG. 21, at act 905, thesoftware resets the timer for the next scheduled interrupt. At act 910,the software services timeouts (i.e., decrease each timeout) and delaysvariables. At act 915, the software executes event-related routines asrequired. At act 920, the software returns from the interrupt to the actit was previously implementing.

[0144] Every time the signal (AcOutHI) crosses zero volts, the microcontroller U1 performs a zero crossing event interrupt. When transistorQ8 (FIG. 12) turns on, it goes into saturation causing a falling edgethat generates an interrupt to the microcontroller U1. The falling edgeis used as a reference edge for activating triacs Q1 and Q2 (FIG.16(b)). When the reference edge occurs, the timer interrupt (FIG. 21) isadjusted so that it will correspond exactly to when a zero crossingoccurs. In this way, the zero crossing interrupt fires the triacs atprecisely the right time.

[0145] To control the power transmitted to the heating elements 235 and240, the microcontroller U1 generates an output signal (first-element orsecond-element) which is provided to the zero-cross triac drivers U5 andU6, respectively. The zero-cross triac drivers U5 and U6 in combinationwith triacs Q1 and Q2 control the high-voltage AC signal (AcIn) beingprovided to the heating elements 235 and 240.

[0146] For controlling the power transmitted to the heating elements 235and 240, triac Q1 or Q2 is fired for a sequence of four sequential halfAC cycles. The triac Q1 or Q2 fired is based on the heating priority andthe status at the software relative to the heating cycle. For example,if the heating priority is “zero-one-hundred”, then only one triac Q2will be fired. Alternatively, if the heating priority is “fifty-fifty”and the heating elements 235 and 240 are being fired sequentially, thenthe software includes a variable specifying which heating element 235 or240 is being activated. After firing a sequence of four sequential AChalf cycles, the software delays firing, i.e. does not fire the triac Q1or Q2 for a number of cycles. The number of cycles the triac Q1 or Q2does not fire is determined by the amount of power to be transmitted tothe heating elements 235 or 240. For example, if 100% power is to betransmitted, then the software will not delay the firing at all. If 50%power is to be transmitted, then the software will delay the firing ofthe triac Q1 or Q2 for four half AC cycles. Table 3 discloses anexemplary power transfer table. TABLE 3 Lookup Table for Various DutyCycles based on an Initial Four Cycle Firing Delay (half-cycle) PowerTransfer 0 half cycle delay 100% Power  1 half cycle delay 80% Power 2half cycle delay 66% Power 3 half cycle delay 57% Power 4 half cycledelay 50% Power 6 half cycle delay 40% Power 16 half cycle delay 20%Power

[0147] Of course, other half cycle delays can be used and the initialfour cycle firing can vary to obtain different power transfer ratios.

[0148] While particular embodiments of the invention have been shown anddescribed herein, changes and modifications may be made withoutdeparting from the spirit and scope of the invention. For example, logicchips other than the Motorola UAA1016A logic chip may be used to controlthe on-off cycle of thyristor 103. Also, a temperature sensing deviceother than the thermistor used as temperature sensing device 102 may beemployed. Also, a thyristor other than a Motorola TRIAC may be used asthyristor 103 and multiple heating elements and other alternativecontrol circuits, as noted above, may be utilized. Therefore, nolimitation of the invention is intended other than limitations containedin the appended claims.

[0149] Various other features and advantages of the invention are setforth in the following claims.

What is claimed is:
 1. A method of controlling a temperature of water ina water heater including a tank for storing water and a heating elementcapable of being powered by a power source, the method comprising theacts of: determining an element characteristic of the heating element;sensing a temperature of the water in the tank; calculating an amount ofpower to be provided to the heating element based at least in part onthe element characteristic and water temperature; and transmitting theamount of power from the power source to the heating element.
 2. Amethod as set forth in claim 1, wherein the element characteristicincludes an element type.
 3. A method as set forth in claim 1, whereinthe element characteristic includes an element wattage.
 4. A method asset forth in claim 1, wherein the element characteristic includes anelement voltage.
 5. A method as set forth in claim 1, wherein theelement characteristic includes the location of the element within thetank.
 6. A method as set forth in claim 1, wherein the water heaterfurther includes a second heating element capable of being powered bythe power source, the method further comprising the acts of: determininga second element characteristic of the second heating element;calculating an amount of power to be provided to the second heatingelement based at least in part on the second element characteristic andthe water temperature; and transmitting the amount of power from thepower source to the second heating element.
 7. A method as set forth inclaim 6, wherein the act of calculating the first amount of power isbased at least in part on the second element characteristic, and whereinthe act of calculating the second amount of power is based in at leastin part on the first element characteristic.
 8. A method as set forth inclaim 1, further comprising: determining a tank characteristic of thetank; and wherein the act of calculating the amount of power is based atleast in part on the tank characteristic.
 9. A method as set forth inclaim 8, wherein the tank characteristic includes a tank capacity.
 10. Amethod as set forth in claim 8, wherein the tank characteristic includesa tank diameter, tank radius, tank circumference or tank cross-sectionalarea.
 11. A method as set forth in claim 8, wherein the tankcharacteristic includes a tank height.
 12. A method as set forth inclaim 1, further comprising: sensing a second temperature of the waterin the tank; and wherein the act of calculating the amount of power isbased at least in part on the second temperature.
 13. A method as setforth in claim 12, wherein the first temperature is sensed by a firstsensor, and wherein the second temperature is sensed by a second sensor.14. A method as set forth in claim 12, wherein the second temperature issensed after the first temperature.
 15. A method as set forth in claim14, further comprising: calculating a slope of a line based at least inpart on the first and second temperatures; and wherein the act ofcalculating the amount of power is based at least in part on thecalculated slope.
 16. A method as set forth in claim 1, furthercomprising: repeating the act of sensing a temperature of the water;storing each sensed temperature, the storing act results in a usagepattern; and wherein the act of calculating an amount of power is basedin part on the usage pattern.
 17. A method as set forth in claim 1,wherein the act of transmitting power comprises: transmitting power fromthe power source to the heating element in a burst.
 18. A method as setforth in claim 6, wherein the act of transmitting power from the powersource to the first element comprises transmitting power from the powersource to the first heating element in a first burst, andw herein theact of transmitting power from the power source to the second elementcomprises transmitting power from the power source to the second heatingelement in a second burst.
 19. A method as set forth in claim 18,wherein the act of transmitting power from the power source to thesecond element in a second burst is after the act of transmitting powerfrom the power source to the first element in a first burst.
 20. Amethod as set forth in claim 1, further comprising the act of: sensingan ambient temperature; and wherein the act of calculating an amount ofpower is based at least in part on the environment temperature.
 21. Amethod as set forth in claim 3, wherein the act of determining anelement characteristic includes the acts of: transmitting a voltage tothe heating element; and calculating an element wattage for the heatingelement.
 22. A method as set forth in claim 11, wherein the act ofcalculating an amount of power includes the acts of: providing a memoryunit having a table comprising a plurality of water heater codes andrespective heating data; creating a first water heater code based atleast in part on the element characteristic; obtaining heating data fromthe table in response to the first water heater code; and calculatingthe amount of power based at least in part on the first water heatercode and the sensed temperature.
 23. A method of controlling atemperature of water in a water heater including a tank for storingwater and a heating element capable of being powered by a power source,the method comprising the acts of: determining a tank characteristic ofthe tank; sensing a temperature of the water in the tank; calculating anamount of power to be provided to the heating element based at least inpart on the tank characteristic and water temperature; and transmittingthe amount of power from the power source to the heating element.
 24. Amethod as set forth in claim 23, wherein the tank characteristicincludes a tank capacity.
 25. A method as set forth in claim 23, whereinthe tank characteristic includes a tank diameter, tank radius, tankcircumference or tank cross-sectional area.
 26. A method as set forth inclaim 23, wherein the tank characteristic includes a tank height.
 27. Amethod as set forth in claim 23, wherein the act of calculating anamount of power includes the acts of: providing a memory unit having atable comprising a plurality of water heater codes and respectiveheating data; creating a first water heater code based at least in parton the tank characteristic; obtaining heating data from the table inresponse to the first water heater code; and calculating the amount ofpower based at least in part on the first water heater code and thesensed temperature.
 28. A method of controlling a temperature of waterin a water heater including a tank for storing water and a heatingelement capable of being powered by a power source, the methodcomprising the acts of: sensing an ambient temperature exterior to thewater heater; sensing a temperature of the water in the tank;calculating an amount of power to be provided to the heating elementbased at least in part on the ambient temperature and the watertemperature; and transmitting the amount of power from the power sourceto the heating element.
 29. A method as set forth in claim 28, themethod further comprising the acts of: determining an elementcharacteristic of the heating element; and wherein the act ofcalculating the amount of power is based at least in part on the elementcharacteristic.
 30. A method as set forth in claim 28, the methodfurther comprising: determining a tank characteristic of the tank; andwherein the act of calculating the amount of power is based at least inpart on the tank characteristic.
 31. A method as set forth in claim 28,further comprising: sensing a second temperature of the water in thetank; and wherein the act of calculating the amount of power is based atleast in part on the second temperature.
 32. A method as set forth inclaim 31, wherein the first temperature is sensed by a first sensor, andwherein the second temperature is sensed by a second sensor.
 33. Amethod as set forth in claim 31, wherein the second temperature issensed after the first temperature.
 34. A method as set forth in claim33, further comprising: calculating a slope of a line based at least inpart on the first and second temperatures; and wherein the act ofcalculating the amount of power is based at least in part on thecalculated slope.
 35. A method as set forth in claim 28, furthercomprising: repeating the act of sensing an ambient temperature; storingeach sensed temperature, the storing act results in a usage pattern; andwherein the act of calculating an amount of power is based in part onthe usage pattern.
 36. A method as set forth in claim 35, furthercomprising: repeating the act of sensing a temperature of the water;storing each sensed ambient temperature.
 37. A method as set forth inclaim 28, wherein the act of transmitting power comprises transmittingpower from the power source to the heating element in a burst.
 38. Asoftware program for generating a signal resulting in an amount of powerto be transmitted to a heating element, the heating element beingdisposed within a tank containing a liquid and the liquid having atemperature being sensed by a temperature sensor, the software programgenerating the signal by: obtaining a water heater code from a memoryunit, the water heater code based at least in part on a one of a heatingelement characteristic and a tank characteristic. receiving thetemperature of the liquid from the temperature sensor; calculating theamount of power to transmit to the heating element based at least inpart on the water heater code and the sensed temperature; and generatinga signal resulting in the amount of power being provided to the heatingelement.
 39. A software program as set forth in claim 38, wherein thewater heater code is based at least in part on the elementcharacteristic and the tank characteristic.
 40. A software program asset forth in claim 38, wherein the element characteristic includes anelement type.
 41. A software program as set forth in claim 38, whereinthe element characteristic includes an element wattage.
 42. A softwareprogram as set forth in claim 38, wherein the element characteristicincludes an element voltage.
 43. A software program as set forth inclaim 38, wherein the element characteristic includes the location ofthe element within the tank.
 44. A software program as set forth inclaim 38, wherein the water heater further includes a second heatingelement capable of being powered by the power source, and wherein thewater heater code is based at least in part on a second heating elementcharacteristic.
 45. A software program as set forth in claim 44, furthercomprising the acts of: calculating a second amount of power to transmitto the second heating element based at least in part on the water heatercode and the sensed temperature; and generating a second signalresulting in the second amount of power being provided to the secondheating element.
 46. A software program as set forth in claim 38,wherein the tank characteristic includes a tank capacity.
 47. A softwareprogram as set forth in claim 38, wherein the tank characteristicincludes a tank diameter, tank radius, tank circumference or tankcross-sectional area.
 48. A software program as set forth in claim 38,wherein the tank characteristic includes a tank height.
 49. A softwareprogram as set forth in claim 38, further comprising: sensing a secondtemperature of the liquid in the tank; and wherein the act ofcalculating the amount of power is based at least in part on the secondtemperature.
 50. A software program as set forth in claim 49, whereinthe second temperature is sensed by a second sensor.
 51. A softwareprogram as set forth in claim 49, wherein the second temperature issensed after the first temperature.
 52. A software program as set forthin claim 51, further comprising: calculating a slope of a line based atleast in part on the first and second temperatures; and wherein the actof calculating the amount of power is based at least in part on thecalculated slope.
 53. A software program as set forth in claim 38,further comprising: repeating the act of sensing a temperature of thewater; storing each sensed temperature, the storing act results in ausage pattern; and wherein the act of calculating an amount of power isbased at least in part on the water usage pattern.
 54. A softwareprogram as set forth in claim 38, wherein the act of generating a signalcomprises: generating a signal resulting in the amount of power beingdelivered to the heating element in a burst with the burst beingfollowed by a period in which no power is delivered to the heatingelement.
 55. A software program as set forth in claim 45, wherein theact of generating a first signal comprises generating a first signalresulting in the first amount of power being delivered to the firstheating element in a first burst with the first burst being followed bya period in which no power is delivered to the first heating element,and wherein the act of generating a second signal comprises generating asecond signal resulting in the second amount of power being delivered tothe second heating element in a second burst with the second burst beingfollowed by a period in which no power is delivered to the secondheating element.
 56. A software program as set forth in claim 55,wherein the act of generating a second signal is after the act ofgenerating a first signal.
 57. A software program as set forth in claim38, further comprising the act of: sensing an environment temperature;and wherein the act of calculating an amount of power is based at leastin part on the environment temperature.
 58. A water heater, comprising:a water tank having a tank characteristic; a water inlet line forintroducing cold water into the tank; a water outlet line capable ofremoving hot water from the tank; a heating element having a heatingsurface disposed within the tank and having an element characteristic; atemperature sensor capable of sensing a temperature of the water withinthe tank; and a controller in communication with the heating element andthe temperature sensor, the controller being operable to receive thesensed temperature from the temperature sensor, to calculate a heatingstrategy for the water heater based at least in part on a one of theelement characteristic and the tank characteristic, and to generate asignal activating the heating element in response to the heatingstrategy.
 59. A water heater as set forth in claim 58, wherein theheating strategy is based at least in part on the element characteristicand the tank characteristic.
 60. A water heater as set forth in claim58, wherein the element characteristic includes an element type.
 61. Awater heater as set forth in claim 58, wherein the elementcharacteristic includes an element wattage.
 62. A water heater as setforth in claim 58, wherein the element characteristic includes anelement voltage.
 63. A water heater as set forth in claim 58, whereinthe element characteristic includes the location of the element withinthe tank.
 64. A water heater as set forth in claim 58, wherein the tankcharacteristic includes a tank capacity.
 65. A water heater as set forthin claim 58, wherein the tank characteristic includes a tank diameter,tank radius, tank circumference or tank cross-sectional area.
 66. Awater heater as set forth in claim 58, wherein the tank characteristicincludes a tank height.
 67. A water heater as set forth in claim 58,further comprising: a second heating element having a second heatingsurface disposed within the tank and having a second elementcharacteristic; and wherein the controller is in communication with theheating element, and wherein the controller is operable to calculate aheating strategy for the water heater based in part on the secondelement characteristic, and to generate a signal activating the secondheating element in response to the heating strategy.
 68. A water heateras set forth in claim 58, wherein the controller is operable to controlthe supply of electric power to the first and second heating elements inbursts, each burst followed by a period during which electric power isnot supplied to the heating element, the control circuit furtheroperable to activate the first heating element or a first time periodand to activate the second heating element for a second time period. 69.A water heater as set forth in claim 59, wherein the second heatingelement is activated after the first heating element is activated.
 70. Awater heater as set forth in claim 59, wherein the first and second timeperiods vary based on the heating strategy.
 71. A storage-type waterheater, comprising: a permanently enclosed water tank to store waterwhile the water is being heated to a set-point temperature; a waterinlet line for adding cold water to the water tank; a water outlet linefor withdrawing heated water from the water tank; at least one electricresistance heating element extending into the water tank to heat waterin the water tank; at least one water temperature sensor operable tosense a water temperature; and a control circuit operable to conductelectric power to the electric resistance heating element in bursts,each burst followed by a period during which electric power is notconducted to the electric resistance heating element, thereby improvingthe efficiency of heating the water in the water tank, and operable tochange the proportion of on to off time in response to the sensed watertemperature and at least one of an element characteristic, a tankcharacteristic, an external water tank temperature, a water consistency,and a history of water use.
 72. A water heater as set forth in claim 71,wherein the element characteristic includes an element type.
 73. A waterheater as set forth in claim 71, wherein the element characteristicincludes an element wattage.
 74. A water heater as set forth in claim71, wherein the element characteristic includes an element voltage. 75.A water heater as set forth in claim 71, wherein the elementcharacteristic includes the location of the element within the tank. 76.A water heater as set forth in claim 71, wherein the tank characteristicincludes a tank capacity.
 77. A water heater as set forth in claim 71,wherein the tank characteristic includes a tank diameter, tank radius,tank circumference or tank cross-sectional area.
 78. A water heater asset forth in claim 71, wherein the tank characteristic includes a tankheight.
 79. A water heater as set forth in claim 71, wherein the controlcircuit includes a microcontroller and a memory unit having a waterheater code, the water heater code based at least in part on a one ofthe element characteristic and the tank characteristic, and wherein themicrocontroller is operable to change the proportion of on to off timein response to at least the sensed water temperature and the waterheater code.
 80. A storage-type water heater, comprising: a permanentlyenclosed water tank to store water while the water is being heated to aset-point temperature; a water inlet line for adding cold water to thewater tank; a water outlet line for withdrawing heated water from thewater tank; at least one electric resistance heating element extendinginto the water tank to heat water in the water tank; at least one watertemperature sensor operable to sense a water temperature; and a controlcircuit capable of conducting electric power to the electric resistanceheating element in bursts, each burst followed by a period during whichelectric power is not conducted to the electric resistance heatingelement, thereby improving the efficiency of heating the water in thewater tank, the control circuit including a microcontroller and softwarefor operating the microcontroller to change the proportion of on to offtime in response to the sensed water temperature and at least one of anelement characteristic, a tank characteristic, an external water tanktemperature, a water consistency, and a history of water use.
 81. Awater heater as set forth in claim 80, wherein the control circuitincludes a memory unit having a water heater code based at least in parton one of the element characteristic and the tank characteristic, andwherein the software operates the microcontroller to change theproportion of on to off time in response to at least the sensed watertemperature and the water heater code.
 82. A water heater as set forthin claim 81, wherein the control circuit includes a memory unit having aheating strategy corresponding to the water heater code, and wherein thesoftware operates the microcontroller to obtain the heating strategy inresponse to the water heater code and to change the proportion of on tooff time in response to at least the sensed temperature and the heatingstrategy.
 83. A water heater as set forth in claim 80, furthercomprising: a second electric resistance heating element extending intothe water tank to heat water in the water tank; and wherein the controlcircuit is capable of conducting electric power to the second electricresistance heating element in bursts, each burst followed by a periodduring which electric power is not conducted to the second electricresistance heating element, thereby improving the efficiency of heatingthe water in the water tank, and wherein the software further operatesthe microcontroller to change the proportion of on to off time for thesecond resistance heating element in response to the sensed watertemperature and at least one of an element characteristic, a tankcharacteristic, an external water tank temperature, a water consistency,and a history of water use.
 84. A water heater as set forth in claim 83,wherein the burst provided to the second electric resistance heatingelement is after the burst provided to the first electric resistanceheating element.